1. Field of the Invention
The present invention relates to a method for manufacturing a rare-earth magnet.
2. Background Art
Rare-earth magnets containing rare-earth elements such as lanthanoide are called permanent magnets, and are used for motors making up a hard disk and a MRI as well as for driving motors for hybrid vehicles, electric vehicles and the like.
Indexes for magnet performance of such rare-earth magnets include remanence (residual flux density) and a coercive force. Meanwhile, as the amount of heat generated at a motor increases because of the trend to more compact motors and higher current density, rare-earth magnets included in the motors also are required to have improved heat resistance, and one of important research challenges in the relating technical field is how to keep a coercive force of a magnet at high temperatures. For example, in the case of a Nd—Fe—B magnet as one of rare-earth magnets that is used often for vehicle driving motors, attempts are made to increase the coercive force of the magnet by developing finer crystal grains, using an alloy having a composition containing Nd more or adding heavy rare-earth elements such as Dy and Tb having a good coercivity performance, for example.
Among heavy rare-earth elements to improve the coercivity performance, Dy is used often for this purpose. However, the amount of deposits of Dy is limited, and Dy is an expensive material. Therefore it is one of important challenges in our country at the nation level to develop a Dy-less magnet to keep the coercivity performance while reducing the amount of Dy or a Dy-free magnet to ensure the coercivity performance without containing Dy at all.
The following briefly describes one example of the method for manufacturing a rare-earth magnet. For instance, Nd—Fe—B molten metal is solidified rapidly to be fine powder, while pressing-forming the fine powder to be a compact. Hot deformation processing is performed to this compact to give magnetic anisotropy thereto to prepare a rare-earth magnet precursor (orientational magnet), into which a modifier alloy is penetrant-diffused to improve the coercive force, thus manufacturing a rare-earth magnet.
Note that JP Patent Publication (Kokai) No. 2011-035001A (Patent Document 1) and JP Patent Publication (Kokai) No. 2010-114200 A (Patent Document 2) disclose a method for manufacturing a rare-earth magnet including a nano-crystalline magnet by adding high-coercivity heavy rare-earth elements by various methods.
The manufacturing method disclosed in Patent Document 1 is to evaporate an evaporation material containing at least one of Dy and Tb so as to be grain-boundary diffused into a hot-deformed compound from a surface thereof.
This manufacturing method requires high-temperature processing at about 850 to 1,050° C. during the evaporation of the evaporation material, and such a temperature range is specified to improve remanence and suppress quick growth of crystal grains.
The heat treatment in the range of as high as 850 to 1,050° C., however, causes grain coarsening, resulting in increase of risk for deterioration in coercive force. That is, even when Dy or Tb is grain-boundary diffused, the resultant may not show a sufficient high coercive force.
Patent Document 2 discloses a manufacturing method that brings at least one type of element of Dy, Tb and Ho or an alloy of the element and at least one type of element of Cu, Al, Ga, Ge, Sn, In, Si, P and Co into contact with the surface of a rare-earth magnet, and performs heat treatment for grain-boundary diffusion so that the grain size does not exceed 1
Patent Document 2 mentions that the temperature range of 500 to 800° C. for heat treatment leads to excellent balance between a diffusion effect of Dy or the like into crystal grain boundary phase and a coarsening suppression effect of crystal grains by the heat treatment, whereby a high coercivity rare-earth magnet can be easily manufactured. Patent Document 2 discloses various embodiments using a Dy—Cu alloy and performing heat treatment at 500 to 900° C. Among these embodiments, a typical 85Dy-15Cu alloy has the melting point of 1,100° C. Accordingly, in order to penetrant-diffuse the molten metal thereof, high-temperature treatment is required at 1,000° C. or higher, and as a result grain coarsening cannot be suppressed.
In view of these circumstances (rise in the price of Dy or the like, grain coarsening under a high-temperature atmosphere to diffuse a modifier alloy containing high-melting heavy rare-earth elements into grain boundary phase or the like), the present inventors have come up with the idea for a method using a modifier alloy (modifier phase) not including heavy rare-earth metals such as Dy and Tb and penetrant-diffusing melt of the modifier alloy under a relatively low-temperature condition, thus manufacturing a high coercivity rare-earth magnet, especially having a high coercive force under a high-temperature atmosphere.